As a power source used in a high frequency heating cooking device such as a microwave oven employed in an ordinary home, a compact and light power source has been desired in view of its quality (to make it portable and a cooking chamber large, the space of a mechanical chamber in which the power source is incorporated is desired to be small). Therefore, the power source has been progressively compact, light and inexpensive by introducing a switching power supply and an inverter power source has been mainly used. Further, a high output is required so that a technology for controlling a large current is necessary. Especially, it is a problem how to suppress the overshoot of an input current generated when the magnetron radiating a microwave begins oscillating from a non-oscillating state and a control system thereof is proposed (for instance, see Patent Document 1).
FIG. 9 shows one example of a power supply for a high frequency heatingpower supply for a high frequency heating (an inverter power source) for driving a magnetron. The power supply for a high frequency heatingpower supply for a high frequency heating includes a dc power source 1, a leakage transformer 2, a first semiconductor switching element 3, a first capacitor 5 (a snubber capacitor), a second capacitor 6 (a resonance capacitor), a third capacitor 7 (a smoothing capacitor), a second semiconductor switching element 4, a driving part 13, a Delon-Greinacher circuit 11 and a magnetron 12.
The dc power source 1 rectifies a commercial power to apply a dc voltage VDC to a series circuit of the second capacitor 6 and a primary winding 8 of the leakage transformer 2. The first semiconductor switching element 3 is connected in series to the second semiconductor switching element 4 and the series circuit of the second capacitor 6 and the primary winding 8 of the leakage transformer 2 is connected in parallel with the second semiconductor switching element 4.
The first capacitor 5 is connected in parallel with the second semiconductor switching element 4 and plays a role of a snubber for suppressing a rush current (voltage) generated during switching. An ac high voltage output generated in a secondary winding 9 of the leakage transformer 2 is converted to a dc high voltage in the Delon-Greinacher circuit 11 and applied to a part between an anode and a cathode of the magnetron 12. A tertiary winding 10 of the leakage transformer 2 supplies a current to the cathode of the magnetron 12.
The first semiconductor switching element 3 and the second semiconductor switching element 4 are composed of IGBTs and free-wheeling diodes connected in parallel therewith. It is to be understood that the first and second semiconductor switching elements 3 and 4 are not limited to this kind and a thyristor, a GTO switching element or the like may be used.
The driving part 13 has therein an oscillating part for forming a driving signal of the first semiconductor switching element 3 and the second semiconductor switching element 4. In this oscillating part, a rectangular wave of a predetermined frequency is generated and a DRIVE signal is supplied pt the first semiconductor switching element 3 and the second semiconductor switching element 4. Immediately after one of the first semiconductor switching element 3 or the second semiconductor switching element 4 is turned off, since the voltage at both ends of the other semiconductor switching element is high, when the semiconductor switching element is turned off at this time, a spike shaped over-current is supplied to generate an unnecessary loss and noise. However, since a dead time is provided so that a turning off operation is delayed until the voltage at both ends is decreased to about 0V, the generation of the unnecessary loss and noise can be prevented. It is to be understood that the same function is realized during an opposite switching operation.
A detailed operation of each mode by the DRIVE signal supplied by the driving part 13 is omitted. As a feature of the circuit structure of FIG. 9, even in 240 V of Europe as the highest voltage in a power source for an ordinary home, a voltage generated in the first semiconductor switching element 3 and the second semiconductor switching element 4 is the same as the dc source voltage VDC, that is, 240√{square root over ( )}2=339V. Accordingly, even when an abnormality such as a lightning surge or an instantaneous voltage drop is assumed to arise, for the first semiconductor switching element 3 and the second semiconductor switching element 4, an inexpensive voltage resistant product of about 600 V can be used without a problem. Further, an input current Iin and a reference voltage (REF) depending on each output level are controlled by an input current constant control part 14, so that the driving part 13 obtains a desired output level.
FIG. 10 shows a state that the magnetron does not oscillate to a state that the magnetron oscillates by the operation of the inverter power source in the input current Iin. Time is shown in an axis of abscissa and the input current Iin(A) and a control signal for the input current (a PWM signal from a microcomputer) are shown in an axis of ordinate on duty. When processes from a non-oscillation to an oscillation of the magnetron are finely classified, 1) a non-oscillation (a start mode), 2) an oscillation (a start mode) and 3) an oscillation (a steady mode) are obtained. Initially, in 1) the non-oscillation (the start mode), under a state of an impedance of infinity that the magnetron does not oscillate, only the input current Iin slightly flows. Accordingly, it is to be understood that a desired input shown by the PWM is not obtained. 2) the oscillation (the start mode) is a part that needs to be improved this time. That is, this part is an area where it is hard that the input current is accurately controlled under the unstable state of the magnetron immediately after the oscillation, and as shown in FIG. 9, an over-shoot is found. In 3) the oscillation (the steady) mode, this area may be said to be an area where a stable input current control can be realized.
Now, FIG. 11 shows resonance characteristics in an inverter power circuit of this kind (a resonance circuit is formed with an inductance L and a capacitance C). FIG. 11 is a diagram showing current-characteristics of working frequency when a constant voltage is applied and frequency f0 indicates a resonance frequency. In an actual operation of the inverter, current-frequency characteristics 11 (a full line part) located within a range of frequencies f1 to f3 higher than the frequency f0 are used.
Namely, at the time of the resonance frequency f0, the current I1 is maximum. As the range of the frequencies is higher toward f1 to f3, the current I1 is more decreased, because as the frequency is lower within the range of f1 to f3, the frequency comes nearer to the resonance frequency, the current supplied to the secondary side of the leakage transformer is increased. On the contrary, when the frequency is higher, the frequency is more remote from the resonance frequency, the current of the secondary side of the leakage transformer is more decreased. In an inverter power source for driving the magnetron as a non-linear load, a desired output is obtained by changing the frequency. For instance, continuous linear outputs that cannot be got in an LC power source can be obtained in such a way that an output is obtained in the vicinity of f3 when 200 W output is used, an output is obtained in the vicinity of f2 when 600 W output is used and an output is obtained in the vicinity of f1 when 1200 W is used. An operating frequency for each output level is supplied by the driving part 13 shown in FIG. 9, however, the contents thereof are realized by the input control constant circuit part 14 that controls the input current converted to voltage to be the same as the reference voltage of each output level. Further, since an ac commercial power source is used, to meet the characteristics of the magnetron that does not oscillate a high frequency when a high voltage is not applied in the vicinities of 0° and 180° of power phases, the operating frequency of the inverter is set, in this section, to a frequency near f1 in which a resonance current is increased. Thus, a boost ratio of magnetron applied voltage to a commercial source voltage can be enhanced and a conductive angle that emits a radio wave can be widened.
Patent Document 1: JP-A-2000-21559